This invention relates to an electric vehicle torque controller that permits efficient use of battery energy.
In the field of electric vehicles, efficient use of battery energy and maximizing the running distance per battery charge are extremely important. In the known conventional art related to this field, as described in Japanese Patent Application Laid-Open No. 250803/1987, a motor magnetic flux loss look-up table is prepared in advance, and an optimum motor magnetic flux signal which reduces the motor loss is read from the table, based on a motor torque signal determined from motor speed and a signal indicative of accelerator pedal depression. A torque current signal is calculated from the motor magnetic flux signal to control the motor accordingly, thereby reducing the motor loss.
Motor torque in an electric motor is proportional to the product of the secondary magnetic flux (motor magnetic flux) generated in the motor (secondary circuit) and the torque current flowing into the secondary circuit. Thus, there are an infinite number of combinations of secondary magnetic flux and torque current values which will yield a particular torque output. However, for each value of torque, there exists one combination which is most efficient in reducing motor losses. Thus, if driving conditions change, the magnetic flux value must be changed in such a manner as to minimize motor losses. When the torque current signal is changed, the torque current flowing into the motor's secondary circuit changes also. Especially when the load or acceleration torque is large, the torque current increases, increasing the torque current sensitivity to external disturbances. The degree of the torque current change against those disturbances is also increased.
In addition, if the magnetic flux signal is changed in response to a change of driving conditions, such as the torque reference signal, motor speed, etc., then the exciting current changes also, causing the secondary magnetic flux generated in the secondary circuit to become unstable. The secondary magnetic flux and the torque current thus interfere with each other, causing the motor torque to oscillate and the vehicle body to vibrate.
One object of the present invention is to provide a torque controller that can generate a required torque output level, with minimum primary current (motor current), and stabilize torque generation in response to a torque reference (a desired torque input value), even when the torque current becomes excessively large, such as, for example, when the vehicle is climbing a steep hill, accelerating abruptly, or operating with a heavy load.